Hitherto, a process for preparing a filler-containing polytetrafluoroethylene (PTFE) granular powder by granulating a mixture of a filler and a PTFE powder with stirring in water was proposed, for example, in JP-B-8611/1968, JP-B-22619/1969, JP-B-37576/1973, JP-B-17855/1974, JP-B-8044/1981, JP-B-18730/1982, etc.
However, by the process for preparation disclosed in each of the above-mentioned patent publications, a filler-containing PTFE granular powder having a small particle size and a sharp particle size distribution could not be obtained.
For that reason, in case of preparing, for example, small articles such as seal rings in the form of a rubber band, thin moldings and molded products having a small surface roughness, there was a problem such that complicated and uneconomical steps must be employed, such as step of sieving a filler-containing PTFE granular powder to take out only particles of small size and molding them, or step of cutting the obtained molding.
Also a granular powder having excellent powder flowability cannot be obtained by only pulverizing a filler-containing PTFE granular powder.
In JP-B-21694/1985, there was proposed a process for preparing a filler-containing PTFE granular powder by granulating, with stirring in water, a PTFE powder and a filler which has been previously surface-treated with an aminosilane compound in coexistence with a water-insoluble organic liquid and an anionic surfactant. However, an apparent density of the filler-containing PTFE granular powder and tensile strength of molded products obtained therefrom are not fully satisfactory.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that those problems can be solved by granulating a mixture of a PTFE powder and a filler in the presence of an organic liquid which forms liquid-liquid interface with water, and a nonionic surfactant with stirring in water.
Namely, an object of the present invention is to provide a filler-containing PTFE granular powder having a good processability and a process for preparation thereof. Particularly an object of the present invention is to provide a filler-containing PTFE granular powder having a large apparent density, a small average particle size, a sharp particle size distribution and good physical properties such as powder flowability, which gives molded products having excellent physical properties such as elongation and surface smoothness, and a process for preparation thereof.